As barriers or safety systems can fail, it is essential to design them well and understand their reliability, which is expressed in an SIL rating. The Safety Integrity Level relates to the Probability of Failure on Demand (PFD). We use complete orders of 10 to describe the reduction factor:
- a SIL 1 protection layer fails maximum in 1 out of 10 demand situations,
- a SIL 2 safety system fails a maximum of once in 100 cases, and
- a SIL 3 once per 1000
SIL classification is well developed for instrumentational safety and described in the ISO IEC 61511 standard. SIL classification of safety systems can be conducted during a LOPA study using the risk matrix or the table from the mentioned standard. The electrical engineer can design and implement the safety system to realize the required integrity level.
A Safety Instrumentation System (SIS) or interlock has a sensor in the process, a logic solver (like a safety PLC), and a final element (e.g., a valve that closes). Automatic protection of the process based on such an instrumentation loop requires regular testing in line with the SIL level.